This invention relates to improvements in double base propellants containing nitrocellulose and nitroglycerine. More particularly, it relates to novel double base propellants of this type possessing improved stability and mesa or plateau type burning characteristics.
As is well known, the burning rate of a propellant at a given initial temperature is mathematically expressed by the equation r = cp.sup.n, where r is the burning rate, p is the pressure and c and n are constants characteristic of the particular propellant composition. This relationship can be expressed as log r = n log p + log c.
For a conventional propellant a plot of log r against log p produces a straight line having a slope n, i.e. the burning rate increases exponentially with increasing pressure. Hence, conventional propellants are not well suited as rocket propellants due to their rapid pressure buildup and dependency of burning rate on initial propellant temperature.
Plateau type propellants, wherein the pressure exponent n is zero in a particular pressure region, partially overcome the aforesaid disadvantages in that at a given ignition temperature they provide a steady burning rate -- and hence steady thrust -- within the region; but their performance is still quite dependent on propellant temperature.
In mesa type propellants the pressure exponent n is negative over a pressure range, so that the burning rate decreases slightly with increasing pressure. Also, unlike conventional and plateau type propellants, mesa propellants are substantially insensitive to temperature over wide ranges of pressure, including the mesa region. Mesa type propellants are thus eminently suited for use in rocket motors, since by virtue of their characteristic of temperature insensitivity, a constant thrust can be obtained by regulating the pressure within the rocket motor by suitable nozzle design to maintain it within the mesa region.
U.S. Pat. No. 3,138,499 discloses nitrocellulose-nitroglycerine double base propellants of the mesa type containing ballistic modifiers comprising lead and/or copper salts of certain aromatic acids, particularly a mixture of lead beta resorcylate and cupric salicylate. Such propellants are characterized, inter alia, by substantially constant burning rates regardless of initial temperature between the pressure ranges of about 200 to over 4000 psi absolute. According to the patent the propellants are made via a "solventless process" by adding the finely ground ballistic modifiers and conventional stabilizers and plasticizers to an agitated aqueous slurry of nitrocellulose and nitroglycerine until a homogeneous mixture is obtained, centrifuging the mass to remove most of the water, aging the resulting paste for 1-5 days at 130.degree. F and thereafter milling the resulting mass to a homogeneous colloid on a heated differential rolling mill.
It has been recently found that instead of incorporating the aforesaid ballistic modifiers into the aqueous slurry of the double base propellant as described in the patent, it is advantageous to heat a mixture of the ballistic modifiers with water or an alcohol independently to produce a reaction product prior to incorporation thereof into the propellant composition, as described in copending U.S. patent application entitled "Ballistic Modifier", Thomas Dunigan et al. inventors, Ser. No. 78,954 filed Oct. 7, 1970. The process provides better control over quality and performance of the ballistic modifier and permits incorporation thereof into the propellant composition by the organic solvent method, which provides important processing advantages over the aqueous slurry method described in the patent.
Unfortunately, the aforesaid propellant composition produced according to the patent, on standing for several months at ordinary temperatures, develops a white crystalline growth on the surface thereof, which acts as an ignition inhibitor. This in turn produces ignition failures, which represent a serious problem with such propellant compositions. The crystalline exudate has been found to consist essentially of beta resorcylic acid, which appears to be the effective ignition inhibitor and is apparently formed by hydrolysis of the lead salt.
By independently preparing a reaction product from lead resorcylate, cupric salicylate and water and incorporating it into the propellant composition by the organic solvent method, as described in the aforesaid patent application, the exudate problem is minimized. However, the problem is still serious when the ratio of the lead salt to copper salt used in said reaction product is greater than unity, which is the more urgent since such ratios include the most effective and desirable ballistic modifier products.